Matter, in simplest terms, is defined as anything that occupies space and has mass. We see examples of matter all around us; trees, human beings, equipment, water, homes, food, etc., everything is matter.
In comparison, a shadow has neither mass nor does it occupy space. It is nothing but a phenomenon created by obstruction of light by an object (one that does not transmit light). A shadow moves with the object (that blocks the path of light) or the source of light. It does not displace anything in its path and hence does not occupy space. It does not have any mass, if it did, it would have caused visible (or noticeable) effects on the objects it falls on. Thus, a shadow can not be classified as matter.
Hope this helps.